There exist large variations in the effective dose of oxidant-producing herbicides (i.e., herbicides that bring about formation of active oxygen species required to kill plants). With bipyridillium herbicides such as paraquat, (1,1'-dimethyl-4-4'bipyridium ion) there exists a considerable variation in susceptibility among weed species. Some species, such as Bromis inermis, Amaranthus retroflexus and wheat, are killed at 0.01 kg/ha, whereas, at the other extreme, 3 kg/ha (i.e., 300 times more) are required for good control of Digitaria. Chenopodium album, Sinapis arvensis and Galium aparine under standard industrial screening conditions (Mr. Pierre Bocion, Dr. R. Maaq Ltd., Dielsdorf, Switzerland, personal communication). Biotypes of certain weed species have evolved resistance to paraquat and are not killed by 16 kg/ha while the sensitive biotype of the same species is controlled by 1kg/ha (Watanabe et al., Weed Research (Japan) 7 49, (1982)). In plants a system is known which detoxifies superoxide normally produced as a dangerous byproduct of photosynthesis. (Scheme 1). ##STR1##
The superoxide dismutating system in plants Such oxygen radicals are naturally produced in low amounts as products of photosynthesis especially at high light intensities (Fover and Halliwell, Planta 133 21 (1976); Nakano and Asada, Plant Cell Physiol. 22 867 (1981); Asada et al., in "Oxidative Damage and Related Enzymes" Harwood Acad. Press, London p.342 (1984)). It has been proposed that superoxide-dismutase converts the oxygen radicals to hydrogen peroxide (which is also toxic) and this in turn is detoxified in a series of steps with ascorbate peroxidase, glutathione reductase and/or dehydroascorbate reductases. Conversely, the peroxide can be chemically converted in the presence of ferrous ions to highly toxic hydroxyl ions by the Fenton reaction. This multiple enzyme system exists in varying levels in different plant species and biotypes. The first enzyme in this sequence (superoxide dismutase) has been reported to be in higher activity levels in tissues of biotypes of species with higher levels of tolerance to paraquat. (Harvey and Harper in Le Baron and Gressel, "Herbicide Resistance in Plants, Wiley p. 215 (1982); Youngman and Dodge, Proc. 5 th Intl. Photosyn. Con. Balahan Intl. Sci., Philadelphia p. 537 (1981); Furusawa et al., Plant Cell Physiol. 25 1247 (1984). These findings have been debated as others, including ourselves, (Fuerst et al., Plant Physiol. 77 984 (1985); Shaaltiel and Gressel, Pestic. Biochem. Physiol. 26 22, (1986), could not repeat their results using the methods stated. Superoxide dismutase is also found in higher activity levels in plants chemically stimulated to higher levels of tolerance to paraquat, an oxidant-generating herbicide (Lewinsohn and Gressel, Plant Physiol. 76 125 (1984)). Plants with a ratio of the antioxidants; ascorbate to alpha-tocopherol of more than 5 and less than 20 were far more tolerant to the photo-oxidant affect of the diphenyl-ether herbicide oxyfluorfen [2-chloro-1(3-ethoxy-4nitrophenoxy)-4-trifluoromethyl)-benzene] (Finckh and Kunert, J. Agric. Food Chem. 33 574 (1985)). Thus ascorbate regenerated by the system can be active in quenching toxic singlet oxygen produced in the presence of some oxidant generating herbicides.
Plants containing higher or lower ratios were more susceptible. Of the enzymes described, it is known that superoxide dismutase contains both copper and zinc (Sawada et al., Biochim. Biophys. Acta 268 305 (1972)). Ascorbate peroxidase, which competes with the Fenton reaction for the peroxide, contains copper and iron (Asada et al., in "Oxidative Damage and Related Enzymes, Harwood Acad. Press p. 342 (1984)). Glutathione reductase and dehydroascorbate reductase are both thiol containing enzymes (Hossain and Asada, Plant Cell Physiol. 25 85 (1984); Halliwell and Foyer, Planta 139 9, (1978)).